Ground Fault Circuit Interrupters (GFCIs) are important safety devices that are common in households and commercial buildings. GFCIs protect users from being electrocuted by monitoring the current flowing in a circuit, and tripping, or opening, the circuit to remove power if an imbalance of current is detected. Conventional GFCIs utilize a solenoid coil to convert electrical energy into mechanical energy in order to trip the device and open one or more sets of electrical contacts. In the conventional arrangement the solenoid comprises a single electrical winding that forms a primary coil having a hollow core with an inner diameter, an outer diameter, a length and a given number of turns of electrical wire. When the solenoid is electrically energized the electrical windings generate a magnetic field that imparts a force upon a plunger located in the hollow core of the solenoid. The plunger in turn moves, and in a conventional GFCI, pushes a spring biased latch mechanism from a latched position to an unlatched position, thereby opening the electrical contacts to remove power from the protected circuit.
The parameters of the solenoid coil are selected to impart a given force upon the plunger that is sufficient to move the latch mechanism. In addition, solenoid coils must be designed with variable operating conditions, such as temperature range, taken into consideration. With higher operating temperatures come higher impedance in the solenoid coil wire, resulting in lower current, smaller magnetic field, and thus lower force imparted on the plunger. Yet another consideration is the need for a failsafe backup operation. If the solenoid coil wire breaks or short circuits, the solenoid can fail to operate or severely reduce the force imparted on the plunger, possibly causing the device not to trip when a fault is detected.
Yet another consideration in the design of solenoid coils is the size of the coil. Typically, solenoids that are required to provide higher force must be made larger to accommodate higher numbers of electrical wire windings. Accordingly, there is a trade-off in the designed force imparted by a solenoid coil and its size. In compact devices the trade-off between size and force capability becomes critical. In particular, Hubbell SnapConnect GFCI devices, which provide a simplified “plug and receptacle” design for connecting a GFCI receptacle to building wiring, have limited internal space as compared to conventional GFCI receptacles, due to the SnapConnect features molded into the housing.
U.S. Pat. No. 1,872,369 to Van Sickle describes a solenoid arranged with three parallel coils and six pins or terminals. The three parallel coils are connected in various arrangements and combinations (parallel and serial) to arrive at a wide variety of pull force, given the same input voltage, or alternately to obtain the same pull force given a different input voltage. The Van Sickle arrangement provides flexibility at the cost of size, and accordingly does not provide a solenoid of reduced size for a given force requirement. The Van Sickle device also does not provide for arranging two or more separate solenoid coils in a manner to enhance the force imparted on a plunger within the solenoid.
U.S. Pat. No. 7,990,663 to Ziegler et al. describes a GFCI device that includes a solenoid coil and an additional “test coil.” The test coil may be energized along with the solenoid coil, but the two coils are not arranged to enhance the force imparted on the plunger. Rather, for example, in one embodiment, the two coils are arranged with opposite polarity, and the test coil is larger than the main coil. Operating both coils together results in the plunger being driven in the opposite direction since the test coil is larger than the primary coil and oriented in the opposite direction. In this manner operation of the solenoid may be confirmed without tripping the contacts. In another embodiment, the test coil is used merely to sense movement of the plunger, and does not enhance the force applied to the plunger. Ziegler does not address the issue of reducing the size of the solenoid coil, but rather adds a second coil used for testing, and accordingly requires additional space within the GFCI housing.
Accordingly, there is a need for an improved solenoid coil, primarily for use in compact GFCI devices, that is smaller in size but still provides the required predetermined mechanical force to trip the device, and that preferably provides back-up capability in the event of a wire break or short circuit in the solenoid winding.